There's also the problem of Lanturn being terribly underleveled for that point in the game. That's a problem for all of the sea denizens, though (barring Magikarp, Wingull and Tentacool, since you get them pretty early on). Chinchou's probably the best of the sea creatures, though. It's offensive and defensive stats are juuuust good enough to keep it alive.
I highly disagree with it being Top, however. Its stats are just good enough, nothing too spectacular, and you get it after Gym 7, which really wrecks its usefulness when you would have really wanted it. Also, remember that being resistant to Glacia's not exactly a big thing since she has that Sheer Cold Walrein that can wreck your day.
I'd prefer Starmie, because at least I'd have something fast to take out Flygon and not get OHKO'd by it. That's a pretty big issue that makes Starmie a much better Ice Beam user than Tentacruel (who was the one I used in my latest RSE adventure) or Lanturn; it outspeeds and OHKOs without risk of OHKO back, also walls Water types easily, also wrecks Skarm, has a field day against Drake (who walks all over Lanturn, truth be told), and also contributes a lot to the fight against Steven (Starmie outspeeds and OHKOs the silly Special Attacking Aggron, scores strong neutral hits against Metagross while outspeeding it, something that Lanturn doesn't do, and scores SE hits on the rest of his team, while Lanturn risks being outsped by Aggron (iirc) and Armaldo, and is outsped and OHKO'd by Metagross).
Sadly, I'd much prefer using Staryu over Chinchou.